void fitSTR1(){
  const double t[] = {0., 2.66956, 5.7895,  13.0694, 23.4691, 33.8689, 49.4686, 67.1483, 91.0678, 118.107, 149.307, 200.};
  const double r[] = {0., 0.42495, 1.04933, 1.44476, 1.94426, 2.44376, 3.15139, 3.81739, 4.3377,  4.6707,  4.89964, 5.};
  static const int n = sizeof(t)/sizeof(double);

  TGraph *g = new TGraph(n, t, r);
  g->SetNameTitle("rt", "space-time relation");
  g->SetMarkerColor(2);
  g->SetMarkerStyle(20);
  TCanvas *c = new TCanvas("c", "calibration of Z", 800, 600);
  TF1 *rt = new TF1("rt", "pol4");
  g->Draw();
  g->Fit(rt);
  cout << "r(0) = " << rt->Eval(0.) << endl;
  TFile *f = new TFile("rt.root", "recreate");
  g->Write();
} // end function fitZ
